1. Field of the Invention
The present invention relates to a thin film electroluminescence device for emitting an electroluminescence (EL) in response to the application of an electric field, and more particularly to a red electroluminescence device comprising a novel combination of host material and luminescent centers.
2. Description of the Prior Art
Since a technique was developed for giving a bright electroluminescence by applying an a.c. electric field to an emitting layer of zinc sulfide (ZnS) doped with an active substance, various investigations have been made on electroluminescence. A thin film electroluminescence (TFEL) device of double-insulated structure is already known which comprises an emitting layer of ZnS, ZnSe or the like doped with Mn as an active substance for providing luminescent centers, a pair of insulating layers sandwiching the emitting layer, and a pair of electrodes sandwiching the assembly and including at least one transparent electrode. This device is now commercially available as a lightweight thin EL display panel because of its high brightness and prolonged life characteristics. Since stable hysteresis (memory effect) can be imparted to the luminescence brightness-applied voltage characteristics of the device by controlling the amount of Mn to be added to the emitting layer, research is under way for its application to the terminals for a multi-purpose input-output devices.
However, there are limitations to the actual use of the above-mentioned TFEL device since the device produces an orange luminescence (585 nm in peak wavelength) only which is specific to the Mn dopant for providing luminescent centers. Presently, therefore, it is strongly desired to realize multicolor luminescence by TFEL devices and expand the market primarily through use in multicolor displays. For multicolor luminescence, LUMOCEN device is proposed wherein luminescent centers of rare-earth fluoride are incorporated in the host material of a ZnS emitting layer in place of Mn (D. Kahng, Appl. Phys. Lett., vol. 13, pp. 210-212, 1968). Research is made on samarium fluoride (SmF.sub.3) and europium fluoride (EuF.sub.3) for use as active substances for red luminescent centers. Nevertheless, rare-earth ions are generally large in ionic radius (at least about 1.0 .ANG.) and are trivalent (Sm.sup.3+, Eu.sup.3+), so that when such ions are added to the ZnS host material, it is difficult to replace Zn lattice points with ions, or the replacement, if possible, impairs the crystallinity of the ZnS host material, owing to the difference in ionic radius (0.75 .ANG. for Zn.sup.2+) as well as in the number of valence electrons (Zn is divalent). This impedes the travel of conduction electrons participating in luminescence, resulting in inefficient luminescence or heat generation of the device. Accordingly, research has been started on TFEL devices wherein the host material is calcium sulfide (CaS), a compound of Ca having an ionic radius (1.000 .ANG.) approximately equal to those of rare-earth elements, as described generally, for example, in Appl. Phys. Lett., vol. 45, pp. 960-961, 1984, etc. However, a satisfactory luminescence brightness still remains to be achieved.